webpki-0.21.4/src/name.rs

// Copyright 2015 Brian Smith.
//
// Permission to use, copy, modify, and/or distribute this software for any
// purpose with or without fee is hereby granted, provided that the above
// copyright notice and this permission notice appear in all copies.
//
// THE SOFTWARE IS PROVIDED "AS IS" AND THE AUTHORS DISCLAIM ALL WARRANTIES
// WITH REGARD TO THIS SOFTWARE INCLUDING ALL IMPLIED WARRANTIES OF
// MERCHANTABILITY AND FITNESS. IN NO EVENT SHALL THE AUTHORS BE LIABLE FOR
// ANY SPECIAL, DIRECT, INDIRECT, OR CONSEQUENTIAL DAMAGES OR ANY DAMAGES
// WHATSOEVER RESULTING FROM LOSS OF USE, DATA OR PROFITS, WHETHER IN AN
// ACTION OF CONTRACT, NEGLIGENCE OR OTHER TORTIOUS ACTION, ARISING OUT OF
// OR IN CONNECTION WITH THE USE OR PERFORMANCE OF THIS SOFTWARE.

use crate::{
    cert::{Cert, EndEntityOrCA},
    der, Error,
};
use core;

/// A DNS Name suitable for use in the TLS Server Name Indication (SNI)
/// extension and/or for use as the reference hostname for which to verify a
/// certificate.
///
/// A `DNSName` is guaranteed to be syntactically valid. The validity rules are
/// specified in [RFC 5280 Section 7.2], except that underscores are also
/// allowed.
///
/// `DNSName` stores a copy of the input it was constructed from in a `String`
/// and so it is only available when the `std` default feature is enabled.
///
/// `Eq`, `PartialEq`, etc. are not implemented because name comparison
/// frequently should be done case-insensitively and/or with other caveats that
/// depend on the specific circumstances in which the comparison is done.
///
/// [RFC 5280 Section 7.2]: https://tools.ietf.org/html/rfc5280#section-7.2
#[cfg(feature = "std")]
#[derive(Clone, Debug, Eq, PartialEq, Hash)]
pub struct DNSName(String);

#[cfg(feature = "std")]
impl DNSName {
    /// Returns a `DNSNameRef` that refers to this `DNSName`.
    pub fn as_ref(&self) -> DNSNameRef { DNSNameRef(self.0.as_bytes()) }
}

#[cfg(feature = "std")]
impl AsRef<str> for DNSName {
    fn as_ref(&self) -> &str { self.0.as_ref() }
}

// Deprecated
#[cfg(feature = "std")]
impl From<DNSNameRef<'_>> for DNSName {
    fn from(dns_name: DNSNameRef) -> Self { dns_name.to_owned() }
}

/// A reference to a DNS Name suitable for use in the TLS Server Name Indication
/// (SNI) extension and/or for use as the reference hostname for which to verify
/// a certificate.
///
/// A `DNSNameRef` is guaranteed to be syntactically valid. The validity rules
/// are specified in [RFC 5280 Section 7.2], except that underscores are also
/// allowed.
///
/// `Eq`, `PartialEq`, etc. are not implemented because name comparison
/// frequently should be done case-insensitively and/or with other caveats that
/// depend on the specific circumstances in which the comparison is done.
///
/// [RFC 5280 Section 7.2]: https://tools.ietf.org/html/rfc5280#section-7.2
#[derive(Clone, Copy)]
pub struct DNSNameRef<'a>(&'a [u8]);

/// An error indicating that a `DNSNameRef` could not built because the input
/// is not a syntactically-valid DNS Name.
#[derive(Clone, Copy, Debug, Eq, PartialEq)]
pub struct InvalidDNSNameError;

impl core::fmt::Display for InvalidDNSNameError {
    fn fmt(&self, f: &mut core::fmt::Formatter) -> core::fmt::Result { write!(f, "{:?}", self) }
}

#[cfg(feature = "std")]
impl ::std::error::Error for InvalidDNSNameError {}

impl<'a> DNSNameRef<'a> {
    /// Constructs a `DNSNameRef` from the given input if the input is a
    /// syntactically-valid DNS name.
    pub fn try_from_ascii(dns_name: &'a [u8]) -> Result<Self, InvalidDNSNameError> {
        if !is_valid_reference_dns_id(untrusted::Input::from(dns_name)) {
            return Err(InvalidDNSNameError);
        }

        Ok(Self(dns_name))
    }

    /// Constructs a `DNSNameRef` from the given input if the input is a
    /// syntactically-valid DNS name.
    pub fn try_from_ascii_str(dns_name: &'a str) -> Result<Self, InvalidDNSNameError> {
        Self::try_from_ascii(dns_name.as_bytes())
    }

    /// Constructs a `DNSName` from this `DNSNameRef`
    #[cfg(feature = "std")]
    pub fn to_owned(&self) -> DNSName {
        // DNSNameRef is already guaranteed to be valid ASCII, which is a
        // subset of UTF-8.
        let s: &str = self.clone().into();
        DNSName(s.to_ascii_lowercase())
    }
}

#[cfg(feature = "std")]
impl core::fmt::Debug for DNSNameRef<'_> {
    fn fmt(&self, f: &mut core::fmt::Formatter) -> Result<(), core::fmt::Error> {
        let lowercase = self.clone().to_owned();
        f.debug_tuple("DNSNameRef").field(&lowercase.0).finish()
    }
}

impl<'a> From<DNSNameRef<'a>> for &'a str {
    fn from(DNSNameRef(d): DNSNameRef<'a>) -> Self {
        // The unwrap won't fail because DNSNameRefs are guaranteed to be ASCII
        // and ASCII is a subset of UTF-8.
        core::str::from_utf8(d).unwrap()
    }
}

pub fn verify_cert_dns_name(
    cert: &super::EndEntityCert, DNSNameRef(dns_name): DNSNameRef,
) -> Result<(), Error> {
    let cert = &cert.inner;
    let dns_name = untrusted::Input::from(dns_name);
    iterate_names(
        cert.subject,
        cert.subject_alt_name,
        Err(Error::CertNotValidForName),
        &|name| {
            match name {
                GeneralName::DNSName(presented_id) =>
                    match presented_dns_id_matches_reference_dns_id(presented_id, dns_name) {
                        Some(true) => {
                            return NameIteration::Stop(Ok(()));
                        },
                        Some(false) => (),
                        None => {
                            return NameIteration::Stop(Err(Error::BadDER));
                        },
                    },
                _ => (),
            }
            NameIteration::KeepGoing
        },
    )
}

// https://tools.ietf.org/html/rfc5280#section-4.2.1.10
pub fn check_name_constraints(
    input: Option<&mut untrusted::Reader>, subordinate_certs: &Cert,
) -> Result<(), Error> {
    let input = match input {
        Some(input) => input,
        None => {
            return Ok(());
        },
    };

    fn parse_subtrees<'b>(
        inner: &mut untrusted::Reader<'b>, subtrees_tag: der::Tag,
    ) -> Result<Option<untrusted::Input<'b>>, Error> {
        if !inner.peek(subtrees_tag as u8) {
            return Ok(None);
        }
        let subtrees = der::nested(inner, subtrees_tag, Error::BadDER, |tagged| {
            der::expect_tag_and_get_value(tagged, der::Tag::Sequence)
        })?;
        Ok(Some(subtrees))
    }

    let permitted_subtrees = parse_subtrees(input, der::Tag::ContextSpecificConstructed0)?;
    let excluded_subtrees = parse_subtrees(input, der::Tag::ContextSpecificConstructed1)?;

    let mut child = subordinate_certs;
    loop {
        iterate_names(child.subject, child.subject_alt_name, Ok(()), &|name| {
            check_presented_id_conforms_to_constraints(name, permitted_subtrees, excluded_subtrees)
        })?;

        child = match child.ee_or_ca {
            EndEntityOrCA::CA(child_cert) => child_cert,
            EndEntityOrCA::EndEntity => {
                break;
            },
        };
    }

    Ok(())
}

fn check_presented_id_conforms_to_constraints(
    name: GeneralName, permitted_subtrees: Option<untrusted::Input>,
    excluded_subtrees: Option<untrusted::Input>,
) -> NameIteration {
    match check_presented_id_conforms_to_constraints_in_subtree(
        name,
        Subtrees::PermittedSubtrees,
        permitted_subtrees,
    ) {
        stop @ NameIteration::Stop(..) => {
            return stop;
        },
        NameIteration::KeepGoing => (),
    };

    check_presented_id_conforms_to_constraints_in_subtree(
        name,
        Subtrees::ExcludedSubtrees,
        excluded_subtrees,
    )
}

#[derive(Clone, Copy)]
enum Subtrees {
    PermittedSubtrees,
    ExcludedSubtrees,
}

fn check_presented_id_conforms_to_constraints_in_subtree(
    name: GeneralName, subtrees: Subtrees, constraints: Option<untrusted::Input>,
) -> NameIteration {
    let mut constraints = match constraints {
        Some(constraints) => untrusted::Reader::new(constraints),
        None => {
            return NameIteration::KeepGoing;
        },
    };

    let mut has_permitted_subtrees_match = false;
    let mut has_permitted_subtrees_mismatch = false;

    loop {
        // http://tools.ietf.org/html/rfc5280#section-4.2.1.10: "Within this
        // profile, the minimum and maximum fields are not used with any name
        // forms, thus, the minimum MUST be zero, and maximum MUST be absent."
        //
        // Since the default value isn't allowed to be encoded according to the
        // DER encoding rules for DEFAULT, this is equivalent to saying that
        // neither minimum or maximum must be encoded.
        fn general_subtree<'b>(
            input: &mut untrusted::Reader<'b>,
        ) -> Result<GeneralName<'b>, Error> {
            let general_subtree = der::expect_tag_and_get_value(input, der::Tag::Sequence)?;
            general_subtree.read_all(Error::BadDER, |subtree| general_name(subtree))
        }

        let base = match general_subtree(&mut constraints) {
            Ok(base) => base,
            Err(err) => {
                return NameIteration::Stop(Err(err));
            },
        };

        let matches = match (name, base) {
            (GeneralName::DNSName(name), GeneralName::DNSName(base)) =>
                presented_dns_id_matches_dns_id_constraint(name, base).ok_or(Error::BadDER),

            (GeneralName::DirectoryName(name), GeneralName::DirectoryName(base)) =>
                presented_directory_name_matches_constraint(name, base, subtrees),

            (GeneralName::IPAddress(name), GeneralName::IPAddress(base)) =>
                presented_ip_address_matches_constraint(name, base),

            // RFC 4280 says "If a name constraints extension that is marked as
            // critical imposes constraints on a particular name form, and an
            // instance of that name form appears in the subject field or
            // subjectAltName extension of a subsequent certificate, then the
            // application MUST either process the constraint or reject the
            // certificate." Later, the CABForum agreed to support non-critical
            // constraints, so it is important to reject the cert without
            // considering whether the name constraint it critical.
            (GeneralName::Unsupported(name_tag), GeneralName::Unsupported(base_tag))
                if name_tag == base_tag =>
                Err(Error::NameConstraintViolation),

            _ => Ok(false),
        };

        match (subtrees, matches) {
            (Subtrees::PermittedSubtrees, Ok(true)) => {
                has_permitted_subtrees_match = true;
            },

            (Subtrees::PermittedSubtrees, Ok(false)) => {
                has_permitted_subtrees_mismatch = true;
            },

            (Subtrees::ExcludedSubtrees, Ok(true)) => {
                return NameIteration::Stop(Err(Error::NameConstraintViolation));
            },

            (Subtrees::ExcludedSubtrees, Ok(false)) => (),

            (_, Err(err)) => {
                return NameIteration::Stop(Err(err));
            },
        }

        if constraints.at_end() {
            break;
        }
    }

    if has_permitted_subtrees_mismatch && !has_permitted_subtrees_match {
        // If there was any entry of the given type in permittedSubtrees, then
        // it required that at least one of them must match. Since none of them
        // did, we have a failure.
        NameIteration::Stop(Err(Error::NameConstraintViolation))
    } else {
        NameIteration::KeepGoing
    }
}

// TODO: document this.
fn presented_directory_name_matches_constraint(
    name: untrusted::Input, constraint: untrusted::Input, subtrees: Subtrees,
) -> Result<bool, Error> {
    match subtrees {
        Subtrees::PermittedSubtrees => Ok(name == constraint),
        Subtrees::ExcludedSubtrees => Ok(true),
    }
}

// https://tools.ietf.org/html/rfc5280#section-4.2.1.10 says:
//
//     For IPv4 addresses, the iPAddress field of GeneralName MUST contain
//     eight (8) octets, encoded in the style of RFC 4632 (CIDR) to represent
//     an address range [RFC4632].  For IPv6 addresses, the iPAddress field
//     MUST contain 32 octets similarly encoded.  For example, a name
//     constraint for "class C" subnet 192.0.2.0 is represented as the
//     octets C0 00 02 00 FF FF FF 00, representing the CIDR notation
//     192.0.2.0/24 (mask 255.255.255.0).
fn presented_ip_address_matches_constraint(
    name: untrusted::Input, constraint: untrusted::Input,
) -> Result<bool, Error> {
    if name.len() != 4 && name.len() != 16 {
        return Err(Error::BadDER);
    }
    if constraint.len() != 8 && constraint.len() != 32 {
        return Err(Error::BadDER);
    }

    // an IPv4 address never matches an IPv6 constraint, and vice versa.
    if name.len() * 2 != constraint.len() {
        return Ok(false);
    }

    let (constraint_address, constraint_mask) = constraint.read_all(Error::BadDER, |value| {
        let address = value.read_bytes(constraint.len() / 2).unwrap();
        let mask = value.read_bytes(constraint.len() / 2).unwrap();
        Ok((address, mask))
    })?;

    let mut name = untrusted::Reader::new(name);
    let mut constraint_address = untrusted::Reader::new(constraint_address);
    let mut constraint_mask = untrusted::Reader::new(constraint_mask);
    loop {
        let name_byte = name.read_byte().unwrap();
        let constraint_address_byte = constraint_address.read_byte().unwrap();
        let constraint_mask_byte = constraint_mask.read_byte().unwrap();
        if ((name_byte ^ constraint_address_byte) & constraint_mask_byte) != 0 {
            return Ok(false);
        }
        if name.at_end() {
            break;
        }
    }

    return Ok(true);
}

#[derive(Clone, Copy)]
enum NameIteration {
    KeepGoing,
    Stop(Result<(), Error>),
}

fn iterate_names(
    subject: untrusted::Input, subject_alt_name: Option<untrusted::Input>,
    result_if_never_stopped_early: Result<(), Error>, f: &dyn Fn(GeneralName) -> NameIteration,
) -> Result<(), Error> {
    match subject_alt_name {
        Some(subject_alt_name) => {
            let mut subject_alt_name = untrusted::Reader::new(subject_alt_name);
            // https://bugzilla.mozilla.org/show_bug.cgi?id=1143085: An empty
            // subjectAltName is not legal, but some certificates have an empty
            // subjectAltName. Since we don't support CN-IDs, the certificate
            // will be rejected either way, but checking `at_end` before
            // attempting to parse the first entry allows us to return a better
            // error code.
            while !subject_alt_name.at_end() {
                let name = general_name(&mut subject_alt_name)?;
                match f(name) {
                    NameIteration::Stop(result) => {
                        return result;
                    },
                    NameIteration::KeepGoing => (),
                }
            }
        },
        None => (),
    }

    match f(GeneralName::DirectoryName(subject)) {
        NameIteration::Stop(result) => result,
        NameIteration::KeepGoing => result_if_never_stopped_early,
    }
}

// It is *not* valid to derive `Eq`, `PartialEq, etc. for this type. In
// particular, for the types of `GeneralName`s that we don't understand, we
// don't even store the value. Also, the meaning of a `GeneralName` in a name
// constraint is different than the meaning of the identically-represented
// `GeneralName` in other contexts.
#[derive(Clone, Copy)]
enum GeneralName<'a> {
    DNSName(untrusted::Input<'a>),
    DirectoryName(untrusted::Input<'a>),
    IPAddress(untrusted::Input<'a>),

    // The value is the `tag & ~(der::CONTEXT_SPECIFIC | der::CONSTRUCTED)` so
    // that the name constraint checking matches tags regardless of whether
    // those bits are set.
    Unsupported(u8),
}

fn general_name<'a>(input: &mut untrusted::Reader<'a>) -> Result<GeneralName<'a>, Error> {
    use ring::io::der::{CONSTRUCTED, CONTEXT_SPECIFIC};
    const OTHER_NAME_TAG: u8 = CONTEXT_SPECIFIC | CONSTRUCTED | 0;
    const RFC822_NAME_TAG: u8 = CONTEXT_SPECIFIC | 1;
    const DNS_NAME_TAG: u8 = CONTEXT_SPECIFIC | 2;
    const X400_ADDRESS_TAG: u8 = CONTEXT_SPECIFIC | CONSTRUCTED | 3;
    const DIRECTORY_NAME_TAG: u8 = CONTEXT_SPECIFIC | CONSTRUCTED | 4;
    const EDI_PARTY_NAME_TAG: u8 = CONTEXT_SPECIFIC | CONSTRUCTED | 5;
    const UNIFORM_RESOURCE_IDENTIFIER_TAG: u8 = CONTEXT_SPECIFIC | 6;
    const IP_ADDRESS_TAG: u8 = CONTEXT_SPECIFIC | 7;
    const REGISTERED_ID_TAG: u8 = CONTEXT_SPECIFIC | 8;

    let (tag, value) = der::read_tag_and_get_value(input)?;
    let name = match tag {
        DNS_NAME_TAG => GeneralName::DNSName(value),
        DIRECTORY_NAME_TAG => GeneralName::DirectoryName(value),
        IP_ADDRESS_TAG => GeneralName::IPAddress(value),

        OTHER_NAME_TAG
        | RFC822_NAME_TAG
        | X400_ADDRESS_TAG
        | EDI_PARTY_NAME_TAG
        | UNIFORM_RESOURCE_IDENTIFIER_TAG
        | REGISTERED_ID_TAG => GeneralName::Unsupported(tag & !(CONTEXT_SPECIFIC | CONSTRUCTED)),

        _ => return Err(Error::BadDER),
    };
    Ok(name)
}

fn presented_dns_id_matches_reference_dns_id(
    presented_dns_id: untrusted::Input, reference_dns_id: untrusted::Input,
) -> Option<bool> {
    presented_dns_id_matches_reference_dns_id_internal(
        presented_dns_id,
        IDRole::ReferenceID,
        reference_dns_id,
    )
}

fn presented_dns_id_matches_dns_id_constraint(
    presented_dns_id: untrusted::Input, reference_dns_id: untrusted::Input,
) -> Option<bool> {
    presented_dns_id_matches_reference_dns_id_internal(
        presented_dns_id,
        IDRole::NameConstraint,
        reference_dns_id,
    )
}

// We do not distinguish between a syntactically-invalid presented_dns_id and
// one that is syntactically valid but does not match reference_dns_id; in both
// cases, the result is false.
//
// We assume that both presented_dns_id and reference_dns_id are encoded in
// such a way that US-ASCII (7-bit) characters are encoded in one byte and no
// encoding of a non-US-ASCII character contains a code point in the range
// 0-127. For example, UTF-8 is OK but UTF-16 is not.
//
// RFC6125 says that a wildcard label may be of the form <x>*<y>.<DNSID>, where
// <x> and/or <y> may be empty. However, NSS requires <y> to be empty, and we
// follow NSS's stricter policy by accepting wildcards only of the form
// <x>*.<DNSID>, where <x> may be empty.
//
// An relative presented DNS ID matches both an absolute reference ID and a
// relative reference ID. Absolute presented DNS IDs are not supported:
//
//      Presented ID   Reference ID  Result
//      -------------------------------------
//      example.com    example.com   Match
//      example.com.   example.com   Mismatch
//      example.com    example.com.  Match
//      example.com.   example.com.  Mismatch
//
// There are more subtleties documented inline in the code.
//
// Name constraints ///////////////////////////////////////////////////////////
//
// This is all RFC 5280 has to say about DNSName constraints:
//
//     DNS name restrictions are expressed as host.example.com.  Any DNS
//     name that can be constructed by simply adding zero or more labels to
//     the left-hand side of the name satisfies the name constraint.  For
//     example, www.host.example.com would satisfy the constraint but
//     host1.example.com would not.
//
// This lack of specificity has lead to a lot of uncertainty regarding
// subdomain matching. In particular, the following questions have been
// raised and answered:
//
//     Q: Does a presented identifier equal (case insensitive) to the name
//        constraint match the constraint? For example, does the presented
//        ID "host.example.com" match a "host.example.com" constraint?
//     A: Yes. RFC5280 says "by simply adding zero or more labels" and this
//        is the case of adding zero labels.
//
//     Q: When the name constraint does not start with ".", do subdomain
//        presented identifiers match it? For example, does the presented
//        ID "www.host.example.com" match a "host.example.com" constraint?
//     A: Yes. RFC5280 says "by simply adding zero or more labels" and this
//        is the case of adding more than zero labels. The example is the
//        one from RFC 5280.
//
//     Q: When the name constraint does not start with ".", does a
//        non-subdomain prefix match it? For example, does "bigfoo.bar.com"
//        match "foo.bar.com"? [4]
//     A: No. We interpret RFC 5280's language of "adding zero or more labels"
//        to mean that whole labels must be prefixed.
//
//     (Note that the above three scenarios are the same as the RFC 6265
//     domain matching rules [0].)
//
//     Q: Is a name constraint that starts with "." valid, and if so, what
//        semantics does it have? For example, does a presented ID of
//        "www.example.com" match a constraint of ".example.com"? Does a
//        presented ID of "example.com" match a constraint of ".example.com"?
//     A: This implementation, NSS[1], and SChannel[2] all support a
//        leading ".", but OpenSSL[3] does not yet. Amongst the
//        implementations that support it, a leading "." is legal and means
//        the same thing as when the "." is omitted, EXCEPT that a
//        presented identifier equal (case insensitive) to the name
//        constraint is not matched; i.e. presented DNSName identifiers
//        must be subdomains. Some CAs in Mozilla's CA program (e.g. HARICA)
//        have name constraints with the leading "." in their root
//        certificates. The name constraints imposed on DCISS by Mozilla also
//        have the it, so supporting this is a requirement for backward
//        compatibility, even if it is not yet standardized. So, for example, a
//        presented ID of "www.example.com" matches a constraint of
//        ".example.com" but a presented ID of "example.com" does not.
//
//     Q: Is there a way to prevent subdomain matches?
//     A: Yes.
//
//        Some people have proposed that dNSName constraints that do not
//        start with a "." should be restricted to exact (case insensitive)
//        matches. However, such a change of semantics from what RFC5280
//        specifies would be a non-backward-compatible change in the case of
//        permittedSubtrees constraints, and it would be a security issue for
//        excludedSubtrees constraints.
//
//        However, it can be done with a combination of permittedSubtrees and
//        excludedSubtrees, e.g. "example.com" in permittedSubtrees and
//        ".example.com" in excludedSubtrees.
//
//     Q: Are name constraints allowed to be specified as absolute names?
//        For example, does a presented ID of "example.com" match a name
//        constraint of "example.com." and vice versa.
//     A: Absolute names are not supported as presented IDs or name
//        constraints. Only reference IDs may be absolute.
//
//     Q: Is "" a valid DNSName constraint? If so, what does it mean?
//     A: Yes. Any valid presented DNSName can be formed "by simply adding zero
//        or more labels to the left-hand side" of "". In particular, an
//        excludedSubtrees DNSName constraint of "" forbids all DNSNames.
//
//     Q: Is "." a valid DNSName constraint? If so, what does it mean?
//     A: No, because absolute names are not allowed (see above).
//
// [0] RFC 6265 (Cookies) Domain Matching rules:
//     http://tools.ietf.org/html/rfc6265#section-5.1.3
// [1] NSS source code:
//     https://mxr.mozilla.org/nss/source/lib/certdb/genname.c?rev=2a7348f013cb#1209
// [2] Description of SChannel's behavior from Microsoft:
//     http://www.imc.org/ietf-pkix/mail-archive/msg04668.html
// [3] Proposal to add such support to OpenSSL:
//     http://www.mail-archive.com/openssl-dev%40openssl.org/msg36204.html
//     https://rt.openssl.org/Ticket/Display.html?id=3562
// [4] Feedback on the lack of clarify in the definition that never got
//     incorporated into the spec:
//     https://www.ietf.org/mail-archive/web/pkix/current/msg21192.html
fn presented_dns_id_matches_reference_dns_id_internal(
    presented_dns_id: untrusted::Input, reference_dns_id_role: IDRole,
    reference_dns_id: untrusted::Input,
) -> Option<bool> {
    if !is_valid_dns_id(presented_dns_id, IDRole::PresentedID, AllowWildcards::Yes) {
        return None;
    }

    if !is_valid_dns_id(reference_dns_id, reference_dns_id_role, AllowWildcards::No) {
        return None;
    }

    let mut presented = untrusted::Reader::new(presented_dns_id);
    let mut reference = untrusted::Reader::new(reference_dns_id);

    match reference_dns_id_role {
        IDRole::ReferenceID => (),

        IDRole::NameConstraint if presented_dns_id.len() > reference_dns_id.len() => {
            if reference_dns_id.len() == 0 {
                // An empty constraint matches everything.
                return Some(true);
            }

            // If the reference ID starts with a dot then skip the prefix of
            // the presented ID and start the comparison at the position of
            // that dot. Examples:
            //
            //                                       Matches     Doesn't Match
            //     -----------------------------------------------------------
            //       original presented ID:  www.example.com    badexample.com
            //                     skipped:  www                ba
            //     presented ID w/o prefix:     .example.com      dexample.com
            //                reference ID:     .example.com      .example.com
            //
            // If the reference ID does not start with a dot then we skip
            // the prefix of the presented ID but also verify that the
            // prefix ends with a dot. Examples:
            //
            //                                       Matches     Doesn't Match
            //     -----------------------------------------------------------
            //       original presented ID:  www.example.com    badexample.com
            //                     skipped:  www                ba
            //                 must be '.':     .                 d
            //     presented ID w/o prefix:      example.com       example.com
            //                reference ID:      example.com       example.com
            //
            if reference.peek(b'.') {
                if presented
                    .skip(presented_dns_id.len() - reference_dns_id.len())
                    .is_err()
                {
                    unreachable!();
                }
            } else {
                if presented
                    .skip(presented_dns_id.len() - reference_dns_id.len() - 1)
                    .is_err()
                {
                    unreachable!();
                }
                if presented.read_byte() != Ok(b'.') {
                    return Some(false);
                }
            }
        },

        IDRole::NameConstraint => (),

        IDRole::PresentedID => unreachable!(),
    }

    // Only allow wildcard labels that consist only of '*'.
    if presented.peek(b'*') {
        if presented.skip(1).is_err() {
            unreachable!();
        }

        loop {
            if reference.read_byte().is_err() {
                return Some(false);
            }
            if reference.peek(b'.') {
                break;
            }
        }
    }

    loop {
        let presented_byte = match (presented.read_byte(), reference.read_byte()) {
            (Ok(p), Ok(r)) if ascii_lower(p) == ascii_lower(r) => p,
            _ => {
                return Some(false);
            },
        };

        if presented.at_end() {
            // Don't allow presented IDs to be absolute.
            if presented_byte == b'.' {
                return None;
            }
            break;
        }
    }

    // Allow a relative presented DNS ID to match an absolute reference DNS ID,
    // unless we're matching a name constraint.
    if !reference.at_end() {
        if reference_dns_id_role != IDRole::NameConstraint {
            match reference.read_byte() {
                Ok(b'.') => (),
                _ => {
                    return Some(false);
                },
            };
        }
        if !reference.at_end() {
            return Some(false);
        }
    }

    assert!(presented.at_end());
    assert!(reference.at_end());

    return Some(true);
}

#[inline]
fn ascii_lower(b: u8) -> u8 {
    match b {
        b'A'..=b'Z' => b + b'a' - b'A',
        _ => b,
    }
}

#[derive(PartialEq)]
enum AllowWildcards {
    No,
    Yes,
}

#[derive(Clone, Copy, PartialEq)]
enum IDRole {
    ReferenceID,
    PresentedID,
    NameConstraint,
}

fn is_valid_reference_dns_id(hostname: untrusted::Input) -> bool {
    is_valid_dns_id(hostname, IDRole::ReferenceID, AllowWildcards::No)
}

// https://tools.ietf.org/html/rfc5280#section-4.2.1.6:
//
//   When the subjectAltName extension contains a domain name system
//   label, the domain name MUST be stored in the dNSName (an IA5String).
//   The name MUST be in the "preferred name syntax", as specified by
//   Section 3.5 of [RFC1034] and as modified by Section 2.1 of
//   [RFC1123].
//
// https://bugzilla.mozilla.org/show_bug.cgi?id=1136616: As an exception to the
// requirement above, underscores are also allowed in names for compatibility.
fn is_valid_dns_id(
    hostname: untrusted::Input, id_role: IDRole, allow_wildcards: AllowWildcards,
) -> bool {
    // https://blogs.msdn.microsoft.com/oldnewthing/20120412-00/?p=7873/
    if hostname.len() > 253 {
        return false;
    }

    let mut input = untrusted::Reader::new(hostname);

    if id_role == IDRole::NameConstraint && input.at_end() {
        return true;
    }

    let mut dot_count = 0;
    let mut label_length = 0;
    let mut label_is_all_numeric = false;
    let mut label_ends_with_hyphen = false;

    // Only presented IDs are allowed to have wildcard labels. And, like
    // Chromium, be stricter than RFC 6125 requires by insisting that a
    // wildcard label consist only of '*'.
    let is_wildcard = allow_wildcards == AllowWildcards::Yes && input.peek(b'*');
    let mut is_first_byte = !is_wildcard;
    if is_wildcard {
        if input.read_byte() != Ok(b'*') || input.read_byte() != Ok(b'.') {
            return false;
        }
        dot_count += 1;
    }

    loop {
        const MAX_LABEL_LENGTH: usize = 63;

        match input.read_byte() {
            Ok(b'-') => {
                if label_length == 0 {
                    return false; // Labels must not start with a hyphen.
                }
                label_is_all_numeric = false;
                label_ends_with_hyphen = true;
                label_length += 1;
                if label_length > MAX_LABEL_LENGTH {
                    return false;
                }
            },

            Ok(b'0'..=b'9') => {
                if label_length == 0 {
                    label_is_all_numeric = true;
                }
                label_ends_with_hyphen = false;
                label_length += 1;
                if label_length > MAX_LABEL_LENGTH {
                    return false;
                }
            },

            Ok(b'a'..=b'z') | Ok(b'A'..=b'Z') | Ok(b'_') => {
                label_is_all_numeric = false;
                label_ends_with_hyphen = false;
                label_length += 1;
                if label_length > MAX_LABEL_LENGTH {
                    return false;
                }
            },

            Ok(b'.') => {
                dot_count += 1;
                if label_length == 0 && (id_role != IDRole::NameConstraint || !is_first_byte) {
                    return false;
                }
                if label_ends_with_hyphen {
                    return false; // Labels must not end with a hyphen.
                }
                label_length = 0;
            },

            _ => {
                return false;
            },
        }
        is_first_byte = false;

        if input.at_end() {
            break;
        }
    }

    // Only reference IDs, not presented IDs or name constraints, may be
    // absolute.
    if label_length == 0 && id_role != IDRole::ReferenceID {
        return false;
    }

    if label_ends_with_hyphen {
        return false; // Labels must not end with a hyphen.
    }

    if label_is_all_numeric {
        return false; // Last label must not be all numeric.
    }

    if is_wildcard {
        // If the DNS ID ends with a dot, the last dot signifies an absolute ID.
        let label_count = if label_length == 0 {
            dot_count
        } else {
            dot_count + 1
        };

        // Like NSS, require at least two labels to follow the wildcard label.
        // TODO: Allow the TrustDomain to control this on a per-eTLD+1 basis,
        // similar to Chromium. Even then, it might be better to still enforce
        // that there are at least two labels after the wildcard.
        if label_count < 3 {
            return false;
        }
    }

    true
}

#[cfg(test)]
mod tests {
    use super::*;

    const PRESENTED_MATCHES_REFERENCE: &[(&[u8], &[u8], Option<bool>)] = &[
        (b"", b"a", None),
        (b"a", b"a", Some(true)),
        (b"b", b"a", Some(false)),
        (b"*.b.a", b"c.b.a", Some(true)),
        (b"*.b.a", b"b.a", Some(false)),
        (b"*.b.a", b"b.a.", Some(false)),
        // Wildcard not in leftmost label
        (b"d.c.b.a", b"d.c.b.a", Some(true)),
        (b"d.*.b.a", b"d.c.b.a", None),
        (b"d.c*.b.a", b"d.c.b.a", None),
        (b"d.c*.b.a", b"d.cc.b.a", None),
        // case sensitivity
        (
            b"abcdefghijklmnopqrstuvwxyz",
            b"ABCDEFGHIJKLMNOPQRSTUVWXYZ",
            Some(true),
        ),
        (
            b"ABCDEFGHIJKLMNOPQRSTUVWXYZ",
            b"abcdefghijklmnopqrstuvwxyz",
            Some(true),
        ),
        (b"aBc", b"Abc", Some(true)),
        // digits
        (b"a1", b"a1", Some(true)),
        // A trailing dot indicates an absolute name, and absolute names can match
        // relative names, and vice-versa.
        (b"example", b"example", Some(true)),
        (b"example.", b"example.", None),
        (b"example", b"example.", Some(true)),
        (b"example.", b"example", None),
        (b"example.com", b"example.com", Some(true)),
        (b"example.com.", b"example.com.", None),
        (b"example.com", b"example.com.", Some(true)),
        (b"example.com.", b"example.com", None),
        (b"example.com..", b"example.com.", None),
        (b"example.com..", b"example.com", None),
        (b"example.com...", b"example.com.", None),
        // xn-- IDN prefix
        (b"x*.b.a", b"xa.b.a", None),
        (b"x*.b.a", b"xna.b.a", None),
        (b"x*.b.a", b"xn-a.b.a", None),
        (b"x*.b.a", b"xn--a.b.a", None),
        (b"xn*.b.a", b"xn--a.b.a", None),
        (b"xn-*.b.a", b"xn--a.b.a", None),
        (b"xn--*.b.a", b"xn--a.b.a", None),
        (b"xn*.b.a", b"xn--a.b.a", None),
        (b"xn-*.b.a", b"xn--a.b.a", None),
        (b"xn--*.b.a", b"xn--a.b.a", None),
        (b"xn---*.b.a", b"xn--a.b.a", None),
        // "*" cannot expand to nothing.
        (b"c*.b.a", b"c.b.a", None),
        // --------------------------------------------------------------------------
        // The rest of these are test cases adapted from Chromium's
        // x509_certificate_unittest.cc. The parameter order is the opposite in
        // Chromium's tests. Also, they some tests were modified to fit into this
        // framework or due to intentional differences between mozilla::pkix and
        // Chromium.
        (b"foo.com", b"foo.com", Some(true)),
        (b"f", b"f", Some(true)),
        (b"i", b"h", Some(false)),
        (b"*.foo.com", b"bar.foo.com", Some(true)),
        (b"*.test.fr", b"www.test.fr", Some(true)),
        (b"*.test.FR", b"wwW.tESt.fr", Some(true)),
        (b".uk", b"f.uk", None),
        (b"?.bar.foo.com", b"w.bar.foo.com", None),
        (b"(www|ftp).foo.com", b"www.foo.com", None), // regex!
        (b"www.foo.com\0", b"www.foo.com", None),
        (b"www.foo.com\0*.foo.com", b"www.foo.com", None),
        (b"ww.house.example", b"www.house.example", Some(false)),
        (b"www.test.org", b"test.org", Some(false)),
        (b"*.test.org", b"test.org", Some(false)),
        (b"*.org", b"test.org", None),
        // '*' must be the only character in the wildcard label
        (b"w*.bar.foo.com", b"w.bar.foo.com", None),
        (b"ww*ww.bar.foo.com", b"www.bar.foo.com", None),
        (b"ww*ww.bar.foo.com", b"wwww.bar.foo.com", None),
        (b"w*w.bar.foo.com", b"wwww.bar.foo.com", None),
        (b"w*w.bar.foo.c0m", b"wwww.bar.foo.com", None),
        (b"wa*.bar.foo.com", b"WALLY.bar.foo.com", None),
        (b"*Ly.bar.foo.com", b"wally.bar.foo.com", None),
        // Chromium does URL decoding of the reference ID, but we don't, and we also
        // require that the reference ID is valid, so we can't test these two.
        //     (b"www.foo.com", b"ww%57.foo.com", Some(true)),
        //     (b"www&.foo.com", b"www%26.foo.com", Some(true)),
        (b"*.test.de", b"www.test.co.jp", Some(false)),
        (b"*.jp", b"www.test.co.jp", None),
        (b"www.test.co.uk", b"www.test.co.jp", Some(false)),
        (b"www.*.co.jp", b"www.test.co.jp", None),
        (b"www.bar.foo.com", b"www.bar.foo.com", Some(true)),
        (b"*.foo.com", b"www.bar.foo.com", Some(false)),
        (b"*.*.foo.com", b"www.bar.foo.com", None),
        // Our matcher requires the reference ID to be a valid DNS name, so we cannot
        // test this case.
        //     (b"*.*.bar.foo.com", b"*..bar.foo.com", Some(false)),
        (b"www.bath.org", b"www.bath.org", Some(true)),
        // Our matcher requires the reference ID to be a valid DNS name, so we cannot
        // test these cases.
        // DNS_ID_MISMATCH("www.bath.org", ""),
        //     (b"www.bath.org", b"20.30.40.50", Some(false)),
        //     (b"www.bath.org", b"66.77.88.99", Some(false)),

        // IDN tests
        (
            b"xn--poema-9qae5a.com.br",
            b"xn--poema-9qae5a.com.br",
            Some(true),
        ),
        (
            b"*.xn--poema-9qae5a.com.br",
            b"www.xn--poema-9qae5a.com.br",
            Some(true),
        ),
        (
            b"*.xn--poema-9qae5a.com.br",
            b"xn--poema-9qae5a.com.br",
            Some(false),
        ),
        (b"xn--poema-*.com.br", b"xn--poema-9qae5a.com.br", None),
        (b"xn--*-9qae5a.com.br", b"xn--poema-9qae5a.com.br", None),
        (b"*--poema-9qae5a.com.br", b"xn--poema-9qae5a.com.br", None),
        // The following are adapted from the examples quoted from
        //   http://tools.ietf.org/html/rfc6125#section-6.4.3
        // (e.g., *.example.com would match foo.example.com but
        // not bar.foo.example.com or example.com).
        (b"*.example.com", b"foo.example.com", Some(true)),
        (b"*.example.com", b"bar.foo.example.com", Some(false)),
        (b"*.example.com", b"example.com", Some(false)),
        (b"baz*.example.net", b"baz1.example.net", None),
        (b"*baz.example.net", b"foobaz.example.net", None),
        (b"b*z.example.net", b"buzz.example.net", None),
        // Wildcards should not be valid for public registry controlled domains,
        // and unknown/unrecognized domains, at least three domain components must
        // be present. For mozilla::pkix and NSS, there must always be at least two
        // labels after the wildcard label.
        (b"*.test.example", b"www.test.example", Some(true)),
        (b"*.example.co.uk", b"test.example.co.uk", Some(true)),
        (b"*.example", b"test.example", None),
        // The result is different than Chromium, because Chromium takes into account
        // the additional knowledge it has that "co.uk" is a TLD. mozilla::pkix does
        // not know that.
        (b"*.co.uk", b"example.co.uk", Some(true)),
        (b"*.com", b"foo.com", None),
        (b"*.us", b"foo.us", None),
        (b"*", b"foo", None),
        // IDN variants of wildcards and registry controlled domains.
        (
            b"*.xn--poema-9qae5a.com.br",
            b"www.xn--poema-9qae5a.com.br",
            Some(true),
        ),
        (
            b"*.example.xn--mgbaam7a8h",
            b"test.example.xn--mgbaam7a8h",
            Some(true),
        ),
        // RFC6126 allows this, and NSS accepts it, but Chromium disallows it.
        // TODO: File bug against Chromium.
        (b"*.com.br", b"xn--poema-9qae5a.com.br", Some(true)),
        (b"*.xn--mgbaam7a8h", b"example.xn--mgbaam7a8h", None),
        // Wildcards should be permissible for 'private' registry-controlled
        // domains. (In mozilla::pkix, we do not know if it is a private registry-
        // controlled domain or not.)
        (b"*.appspot.com", b"www.appspot.com", Some(true)),
        (b"*.s3.amazonaws.com", b"foo.s3.amazonaws.com", Some(true)),
        // Multiple wildcards are not valid.
        (b"*.*.com", b"foo.example.com", None),
        (b"*.bar.*.com", b"foo.bar.example.com", None),
        // Absolute vs relative DNS name tests. Although not explicitly specified
        // in RFC 6125, absolute reference names (those ending in a .) should
        // match either absolute or relative presented names.
        // TODO: File errata against RFC 6125 about this.
        (b"foo.com.", b"foo.com", None),
        (b"foo.com", b"foo.com.", Some(true)),
        (b"foo.com.", b"foo.com.", None),
        (b"f.", b"f", None),
        (b"f", b"f.", Some(true)),
        (b"f.", b"f.", None),
        (b"*.bar.foo.com.", b"www-3.bar.foo.com", None),
        (b"*.bar.foo.com", b"www-3.bar.foo.com.", Some(true)),
        (b"*.bar.foo.com.", b"www-3.bar.foo.com.", None),
        // We require the reference ID to be a valid DNS name, so we cannot test this
        // case.
        //     (b".", b".", Some(false)),
        (b"*.com.", b"example.com", None),
        (b"*.com", b"example.com.", None),
        (b"*.com.", b"example.com.", None),
        (b"*.", b"foo.", None),
        (b"*.", b"foo", None),
        // The result is different than Chromium because we don't know that co.uk is
        // a TLD.
        (b"*.co.uk.", b"foo.co.uk", None),
        (b"*.co.uk.", b"foo.co.uk.", None),
    ];

    #[test]
    fn presented_matches_reference_test() {
        for &(presented, reference, expected_result) in PRESENTED_MATCHES_REFERENCE {
            use std::string::String;

            let actual_result = presented_dns_id_matches_reference_dns_id(
                untrusted::Input::from(presented),
                untrusted::Input::from(reference),
            );
            assert_eq!(
                actual_result,
                expected_result,
                "presented_dns_id_matches_reference_dns_id(\"{}\", IDRole::ReferenceID, \"{}\")",
                String::from_utf8_lossy(presented),
                String::from_utf8_lossy(reference)
            );
        }
    }
}